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Huang D, Song CL, Webb TA, Fang S, Chang CZ, Moodera JS, Kaxiras E, Hoffman JE. Revealing the Empty-State Electronic Structure of Single-Unit-Cell FeSe/SrTiO3. PHYSICAL REVIEW LETTERS 2015; 115:017002. [PMID: 26182116 DOI: 10.1103/physrevlett.115.017002] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/17/2015] [Indexed: 06/04/2023]
Abstract
We use scanning tunneling spectroscopy to investigate the filled and empty electronic states of superconducting single-unit-cell FeSe deposited on SrTiO3(001). We map the momentum-space band structure by combining quasiparticle interference imaging with decay length spectroscopy. In addition to quantifying the filled-state bands, we discover a Γ-centered electron pocket 75 meV above the Fermi energy. Our density functional theory calculations show the orbital nature of empty states at Γ and explain how the Se height is a key tuning parameter of their energies, with broad implications for electronic properties.
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Chang CZ, Zhao W, Kim DY, Zhang H, Assaf BA, Heiman D, Zhang SC, Liu C, Chan MHW, Moodera JS. High-precision realization of robust quantum anomalous Hall state in a hard ferromagnetic topological insulator. NATURE MATERIALS 2015; 14:473-7. [PMID: 25730394 DOI: 10.1038/nmat4204] [Citation(s) in RCA: 250] [Impact Index Per Article: 27.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/14/2014] [Accepted: 12/19/2014] [Indexed: 05/13/2023]
Abstract
The discovery of the quantum Hall (QH) effect led to the realization of a topological electronic state with dissipationless currents circulating in one direction along the edge of a two-dimensional electron layer under a strong magnetic field. The quantum anomalous Hall (QAH) effect shares a similar physical phenomenon to that of the QH effect, whereas its physical origin relies on the intrinsic spin-orbit coupling and ferromagnetism. Here, we report the experimental observation of the QAH state in V-doped (Bi,Sb)2Te3 films with the zero-field longitudinal resistance down to 0.00013 ± 0.00007h/e(2) (~3.35 ± 1.76 Ω), Hall conductance reaching 0.9998 ± 0.0006e(2)/h and the Hall angle becoming as high as 89.993° ± 0.004° at T = 25 mK. A further advantage of this system comes from the fact that it is a hard ferromagnet with a large coercive field (Hc > 1.0 T) and a relative high Curie temperature. This realization of a robust QAH state in hard ferromagnetic topological insulators (FMTIs) is a major step towards dissipationless electronic applications in the absence of external fields.
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Li M, Chang CZ, Wu L, Tao J, Zhao W, Chan MHW, Moodera JS, Li J, Zhu Y. Experimental verification of the van Vleck nature of long-range ferromagnetic order in the vanadium-doped three-dimensional topological insulator Sb(2)Te(3). PHYSICAL REVIEW LETTERS 2015; 114:146802. [PMID: 25910148 DOI: 10.1103/physrevlett.114.146802] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/18/2014] [Indexed: 06/04/2023]
Abstract
We demonstrate by high resolution low temperature electron energy loss spectroscopy (EELS) measurements that the long range ferromagnetic (FM) order in the vanadium- (V-)doped topological insulator Sb_{2}Te_{3} has the nature of van Vleck-type ferromagnetism. The positions and the relative amplitudes of two core-level peaks (L_{3} and L_{2}) of the V EELS spectrum show unambiguous change when the sample is cooled from room temperature to T=10 K. Magnetotransport and comparison of the measured and simulated EELS spectra confirm that these changes originate from the onset of FM order. Crystal field analysis indicates that in V-doped Sb_{2}Te_{3}, partially filled core states contribute to the FM order. Since van Vleck magnetism is a result of summing over all states, this magnetization of core level verifies the van Vleck-type ferromagnetism in a direct manner.
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Miao GX, Moodera JS. Spin manipulation with magnetic semiconductor barriers. Phys Chem Chem Phys 2015; 17:751-61. [DOI: 10.1039/c4cp04599h] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Magnetic semiconductors with unique spin-filtering property and the ability to create excessive internal magnetic fields can open myriads of new phenomena.
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Ciudad D, Gobbi M, Kinane CJ, Eich M, Moodera JS, Hueso LE. Sign control of magnetoresistance through chemically engineered interfaces. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2014; 26:7561-7567. [PMID: 25339373 DOI: 10.1002/adma.201401283] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/22/2014] [Revised: 08/11/2014] [Indexed: 06/04/2023]
Abstract
Chemically engineered interfaces are shown to produce inversions of the magnetoresistance in spintronic devices including lithium fluoride interlayers. This behavior is explained by the formation of anti-ferromagnetic difluoride layers. By changing the order of deposition of the different materials, the sign of the magnetoresistance can be deterministically controlled both in organic spin valves and in inorganic magnetic tunnel junctions.
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Wei P, Katmis F, Assaf BA, Steinberg H, Jarillo-Herrero P, Heiman D, Moodera JS. Exchange-coupling-induced symmetry breaking in topological insulators. PHYSICAL REVIEW LETTERS 2013; 110:186807. [PMID: 23683236 DOI: 10.1103/physrevlett.110.186807] [Citation(s) in RCA: 90] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/20/2012] [Indexed: 05/22/2023]
Abstract
An exchange gap in the Dirac surface states of a topological insulator (TI) is necessary for observing the predicted unique features such as the topological magnetoelectric effect as well as to confine Majorana fermions. We experimentally demonstrate proximity-induced ferromagnetism in a TI, combining a ferromagnetic insulator EuS layer with Bi(2)Se(3), without introducing defects. By magnetic and magnetotransport studies, including anomalous Hall effect and magnetoresistance measurements, we show the emergence of a ferromagnetic phase in TI, a step forward in unveiling their exotic properties.
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Raman KV, Kamerbeek AM, Mukherjee A, Atodiresei N, Sen TK, Lazić P, Caciuc V, Michel R, Stalke D, Mandal SK, Blügel S, Münzenberg M, Moodera JS. Interface-engineered templates for molecular spin memory devices. Nature 2013; 493:509-13. [PMID: 23344361 DOI: 10.1038/nature11719] [Citation(s) in RCA: 216] [Impact Index Per Article: 19.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/11/2012] [Accepted: 10/23/2012] [Indexed: 11/09/2022]
Abstract
The use of molecular spin state as a quantum of information for storage, sensing and computing has generated considerable interest in the context of next-generation data storage and communication devices, opening avenues for developing multifunctional molecular spintronics. Such ideas have been researched extensively, using single-molecule magnets and molecules with a metal ion or nitrogen vacancy as localized spin-carrying centres for storage and for realizing logic operations. However, the electronic coupling between the spin centres of these molecules is rather weak, which makes construction of quantum memory registers a challenging task. In this regard, delocalized carbon-based radical species with unpaired spin, such as phenalenyl, have shown promise. These phenalenyl moieties, which can be regarded as graphene fragments, are formed by the fusion of three benzene rings and belong to the class of open-shell systems. The spin structure of these molecules responds to external stimuli (such as light, and electric and magnetic fields), which provides novel schemes for performing spin memory and logic operations. Here we construct a molecular device using such molecules as templates to engineer interfacial spin transfer resulting from hybridization and magnetic exchange interaction with the surface of a ferromagnet; the device shows an unexpected interfacial magnetoresistance of more than 20 per cent near room temperature. Moreover, we successfully demonstrate the formation of a nanoscale magnetic molecule with a well-defined magnetic hysteresis on ferromagnetic surfaces. Owing to strong magnetic coupling with the ferromagnet, such independent switching of an adsorbed magnetic molecule has been unsuccessful with single-molecule magnets. Our findings suggest the use of chemically amenable phenalenyl-based molecules as a viable and scalable platform for building molecular-scale quantum spin memory and processors for technological development.
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Li B, Roschewsky N, Assaf BA, Eich M, Epstein-Martin M, Heiman D, Münzenberg M, Moodera JS. Superconducting spin switch with infinite magnetoresistance induced by an internal exchange field. PHYSICAL REVIEW LETTERS 2013; 110:097001. [PMID: 23496737 DOI: 10.1103/physrevlett.110.097001] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/18/2012] [Indexed: 06/01/2023]
Abstract
A theoretical prediction by de Gennes suggests that the resistance in a FI/S/FI (where FI is a ferromagnetic insulator, and S is a superconductor) structure will depend on the magnetization direction of the two FI layers. We report a magnetotransport measurement in a EuS/Al/EuS structure, showing that an infinite magnetoresistance can be produced by tuning the internal exchange field at the FI/S interface. This proximity effect at the interface can be suppressed by an Al(2)O(3) barrier as thin as 0.3 nm, showing the extreme confinement of the interaction to the interface giving rise to the demonstrated phenomena.
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Assaf BA, Cardinal T, Wei P, Katmis F, Moodera JS, Heiman D. Modified electrical transport probe design for standard magnetometer. THE REVIEW OF SCIENTIFIC INSTRUMENTS 2012; 83:033904. [PMID: 22462935 DOI: 10.1063/1.3697998] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Making electrical transport measurements on a material is often a time consuming process that involves testing a large number of samples. It is thus inconvenient to wire up and rewire samples onto a sample probe. We therefore present a method of modifying Quantum Design's MPMS SQUID magnetometer transport probe that simplifies the process of sample mounting. One of the difficulties to overcome is the small diameter of the sample space. A small socket is designed and mounted on the probe so that various samples mounted on individual headers can be readily exchanged in the socket. We also present some test results on the topological insulator Bi(2)Te(2)Se using the modified probe.
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Walter M, Walowski J, Zbarsky V, Münzenberg M, Schäfers M, Ebke D, Reiss G, Thomas A, Peretzki P, Seibt M, Moodera JS, Czerner M, Bachmann M, Heiliger C. Seebeck effect in magnetic tunnel junctions. NATURE MATERIALS 2011; 10:742-746. [PMID: 21785418 DOI: 10.1038/nmat3076] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/14/2011] [Accepted: 06/20/2011] [Indexed: 05/31/2023]
Abstract
Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, that is, the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge-based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In this respect, it is the analogue to the tunnelling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configurations are of the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. The geometric centre of the electronic density of states relative to the Fermi level determines the size of the Seebeck effect. Experimentally, we realized 8.8% magneto-Seebeck effect, which results from a voltage change of about -8.7 μV K⁻¹ from the antiparallel to the parallel direction close to the predicted value of -12.1 μV K⁻¹. In contrast to the spin-Seebeck effect, it can be measured as a voltage change directly without conversion of a spin current.
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Sierra JF, Pryadun VV, Russek SE, García-Hernández M, Mompean F, Rozada R, Chubykalo-Fesenko O, Snoeck E, Miao GX, Moodera JS, Aliev FG. Interface and temperature dependent magnetic properties in permalloy thin films and tunnel junction structures. JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY 2011; 11:7653-7664. [PMID: 22097470 DOI: 10.1166/jnn.2011.4754] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
Magnetization dynamics and field dependent magnetization of different devices based on 25-30 nm thick Permalloy (Py) films: such as single Py layers (Py/MgO; Py/CoFeB/Al2O3) and Py inserted as a magnetic layer in magnetic tunnel junctions (Py/CoFe/Al2O3/CoFe; Py/CoFeB/Al2O3/CoFe; Py/MgO/Fe) have been extensively studied within a temperature range between 300 K down to 5 K. The dynamic response was investigated in the linear regime measuring the ferromagnetic resonance response of the Py layers using broadband vector network analyzer technique. Both the static and the dynamic properties suggest the possible presence of a thermally induced spin reorientation transition in the Py interface at temperatures around 60 K in all the samples investigated. It seems, however, that the details of the interface between Py and the hardening ferromagnet/insulator structure, the atomic structure of Py layers (amorphous vs. textured) as well as the presence of dipolar coupling through the insulating barrier in the magnetic tunnel junction structures could strongly influence this low temperature reorientation transition. Our conclusions are indirectly supported by structural characterization of the samples by means of X-Ray diffraction and high resolution transmission electron microscopy techniques. Micromagnetic simulations indicate the possibility of strongly enhanced surface anisotropy in thin Py films over CoFe or CoFeB underlayers. Comparison of the simulations with experimental results also shows that the thermally-induced spin reorientation transition could be influenced by the presence of strong disorder at the surface.
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Park YJ, Hickey MC, Van Veenhuizen MJ, Chang J, Heiman D, Perry CH, Moodera JS. Efficient spin transfer phenomena in Fe/MgO/GaAs structure. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2011; 23:116002. [PMID: 21358034 DOI: 10.1088/0953-8984/23/11/116002] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
The efficiency of spin polarized charge transfer was investigated in an Fe/MgO tunnel barrier/GaAs based structure using spin dependent photocurrent measurements, whereby a spin imbalance in carrier population was generated in the GaAs by circularly polarized light. The dominance of tunneling transport processes over Schottky emission gave rise to a high spin transfer efficiency of 35% under the photovoltaic mode of device operation. A spin dependent tunneling conductance associated with spin polarized electron transport was identified by the observation of phase changes. This transport prevails over the unpolarized electron and hole conduction over the bias range which corresponds to flat band conditions.
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Moodera JS, Kim TH, Tanaka C, De Groot CH. Spin-polarized tunnelling, magnetoresistance and interfacial effects in ferromagnetic junctions. ACTA ACUST UNITED AC 2009. [DOI: 10.1080/13642810008208608] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
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Mohite AD, Santos TS, Moodera JS, Alphenaar BW. Observation of the triplet exciton in EuS-coated single-walled nanotubes. NATURE NANOTECHNOLOGY 2009; 4:425-429. [PMID: 19581894 DOI: 10.1038/nnano.2009.122] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/23/2009] [Accepted: 04/28/2009] [Indexed: 05/28/2023]
Abstract
Photon absorption by carbon nanotubes creates bound electron-hole pairs called excitons, which can exist in spin-polarized triplet or spin-unpolarized singlet configurations. Triplet excitons are optically inactive owing to the weak spin-orbit coupling in nanotubes. This prevents the optical injection of electron spin into nanotubes for spintronic applications and limits the efficiency of photocurrent generation. Here, we show that it is possible to optically excite the triplet exciton by using a ferromagnetic semiconductor as a spin filter to mix the singlet and triplet excitons. The triplet contribution to the photocurrent is detected, representing the first direct evidence of the triplet exciton in carbon nanotubes.
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Hickey MC, Moodera JS. Origin of intrinsic Gilbert damping. PHYSICAL REVIEW LETTERS 2009; 102:137601. [PMID: 19392403 DOI: 10.1103/physrevlett.102.137601] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/17/2008] [Indexed: 05/27/2023]
Abstract
The damping of magnetization, represented by the rate at which it relaxes to equilibrium, is successfully modeled as a phenomenological extension in the Landau-Lifschitz-Gilbert equation. This is the damping torque term known as Gilbert damping and its direction is given by the vector product of the magnetization and its time derivative. Here we derive the Gilbert term from first-principles by a nonrelativistic expansion of the Dirac equation. We find that this term arises when one calculates the time evolution of the spin observable in the presence of the full spin-orbital coupling terms, while recognizing the relationship between the curl of the electric field and the time-varying magnetic induction.
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Miao GX, Müller M, Moodera JS. Magnetoresistance in double spin filter tunnel junctions with nonmagnetic electrodes and its unconventional bias dependence. PHYSICAL REVIEW LETTERS 2009; 102:076601. [PMID: 19257701 DOI: 10.1103/physrevlett.102.076601] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/18/2008] [Indexed: 05/27/2023]
Abstract
Spin filtering happens due to the discriminative tunneling probabilities for spin-up and spin-down electrons through a magnetic barrier and can result in highly spin polarized tunnel currents. Combining two such barriers in a tunnel junction thus leads to large magnetoresistance without the necessity of magnetic electrodes. We demonstrate the realization of such unconventional tunnel junctions using double EuS spin filter barriers with Al electrodes. The novel nonmonotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations.
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Müller GM, Walowski J, Djordjevic M, Miao GX, Gupta A, Ramos AV, Gehrke K, Moshnyaga V, Samwer K, Schmalhorst J, Thomas A, Hütten A, Reiss G, Moodera JS, Münzenberg M. Spin polarization in half-metals probed by femtosecond spin excitation. NATURE MATERIALS 2009; 8:56-61. [PMID: 19079243 DOI: 10.1038/nmat2341] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/30/2008] [Accepted: 10/21/2008] [Indexed: 05/27/2023]
Abstract
Knowledge of the spin polarization is of fundamental importance for the use of a material in spintronics applications. Here, we used femtosecond optical excitation of half-metals to distinguish between half-metallic and metallic properties. Because the direct energy transfer by Elliot-Yafet scattering is blocked in a half-metal, the demagnetization time is a measure for the degree of half-metallicity. We propose that this characteristic enables us vice versa to establish a novel and fast characterization tool for this highly important material class used in spin-electronic devices. The technique has been applied to a variety of materials where the spin polarization at the Fermi level ranges from 45 to 98%: Ni, Co(2)MnSi, Fe(3)O(4), La(0.66)Sr(0.33)MnO(3) and CrO(2).
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Santos TS, Moodera JS, Raman KV, Negusse E, Holroyd J, Dvorak J, Liberati M, Idzerda YU, Arenholz E. Determining exchange splitting in a magnetic semiconductor by spin-filter tunneling. PHYSICAL REVIEW LETTERS 2008; 101:147201. [PMID: 18851564 DOI: 10.1103/physrevlett.101.147201] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/24/2008] [Indexed: 05/26/2023]
Abstract
A large exchange splitting of the conduction band in ultrathin films of the ferromagnetic semiconductor EuO was determined quantitatively, by using EuO as a tunnel barrier and fitting the current-voltage characteristics and temperature dependence to tunneling theory. This exchange splitting leads to different tunnel barrier heights for spin-up and spin-down electrons and is large enough to produce a near-fully spin-polarized current. Moreover, the magnetic properties of these ultrathin films (<6 nm) show a reduction in Curie temperature with decreasing thickness, in agreement with theoretical calculation [R. Schiller, Phys. Rev. Lett. 86, 3847 (2001)10.1103/Phys. Rev. Lett.86.3847].
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Miao GX, Ramos AV, Moodera JS. Infinite magnetoresistance from the spin dependent proximity effect in symmetry driven bcc-Fe/V/Fe heteroepitaxial superconducting spin valves. PHYSICAL REVIEW LETTERS 2008; 101:137001. [PMID: 18851482 DOI: 10.1103/physrevlett.101.137001] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/13/2008] [Indexed: 05/26/2023]
Abstract
Superconductivity in fully epitaxial bcc-Fe/V/Fe hybrid spin valve structures is influenced by the spin currents and supercurrents as well as band symmetry. The transition temperature is spin dependent in the presence of the proximity effect. A unique feature in this system is the band symmetry filtering taking place at the Fe/V interface. The absence of Delta2 Bloch states at the Fermi level in the Fe spin majority channel leads to spin selectivity and reduced transparency at the interface. Infinite magnetoresistance with clear remanence states is obtained, and implies the potential for spintronic applications.
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Miao GX, Park YJ, Moodera JS, Seibt M, Eilers G, Münzenberg M. Disturbance of tunneling coherence by oxygen vacancy in epitaxial Fe/MgO/Fe magnetic tunnel junctions. PHYSICAL REVIEW LETTERS 2008; 100:246803. [PMID: 18643609 DOI: 10.1103/physrevlett.100.246803] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/04/2008] [Indexed: 05/26/2023]
Abstract
Oxygen vacancies in the MgO barriers of epitaxial Fe/MgO/Fe magnetic tunnel junctions are observed to introduce symmetry-breaking scatterings and hence open up channels for noncoherent tunneling processes that follow the normal WKB approximation. The evanescent waves inside the MgO barrier thus experience two-step tunneling, the coherent followed by the noncoherent process, and lead to lower tunnel magnetoresistance, higher junction resistance, as well as increased bias and temperature dependence. The characteristic length of the symmetry scattering process is determined to be about 1.6 nm.
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Shim JH, Raman KV, Park YJ, Santos TS, Miao GX, Satpati B, Moodera JS. Large spin diffusion length in an amorphous organic semiconductor. PHYSICAL REVIEW LETTERS 2008; 100:226603. [PMID: 18643439 DOI: 10.1103/physrevlett.100.226603] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/20/2008] [Indexed: 05/26/2023]
Abstract
We directly measured a spin diffusion length (lambdas) of 13.3 nm in amorphous organic semiconductor (OS) rubrene (C42H28) by spin polarized tunneling. In comparison, no spin-conserved transport has been reported in amorphous Si or Ge. Absence of dangling bond defects can explain the spin transport behavior in amorphous OS. Furthermore, when rubrene barriers were grown on a seed layer, the elastic tunneling characteristics were greatly enhanced. Based on our findings, lambdas in single-crystalline rubrene can be expected to reach even millimeters, showing the potential for organic spintronics development.
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Nagahama T, Santos TS, Moodera JS. Enhanced magnetotransport at high bias in quasimagnetic tunnel junctions with EuS spin-filter barriers. PHYSICAL REVIEW LETTERS 2007; 99:016602. [PMID: 17678177 DOI: 10.1103/physrevlett.99.016602] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/06/2007] [Indexed: 05/16/2023]
Abstract
In quasimagnetic tunnel junctions with a EuS spin-filter tunnel barrier between Al and Co electrodes, we observed large magnetoresistance (MR). The bias dependence shows an abrupt increase of MR ratio in high bias voltage, which is contrary to conventional magnetic tunnel junctions. This behavior can be understood as due to Fowler-Nordheim tunneling through the fully spin-polarized EuS conduction band. The I-V characteristics and bias dependence of MR calculated using tunneling theory show excellent agreement with experiment.
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Miao GX, Yoon K, Santos TS, Moodera JS. Influence of spin-polarized current on superconductivity and the realization of large magnetoresistance. PHYSICAL REVIEW LETTERS 2007; 98:267001. [PMID: 17678118 DOI: 10.1103/physrevlett.98.267001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/10/2006] [Indexed: 05/16/2023]
Abstract
The superconducting state can be influenced by injecting spin-polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance of over 1100% for a F/I/S/I/F multilayer system (I insulator). Because of the competition between ferromagnetism and superconductivity, the superconducting transition temperature (T(C)) in the spin-parallel configuration is shifted below that in the spin antiparallel configuration. The T(C) shift is attributed to ferromagnet-induced nonequilibrium spin carriers in the superconductors.
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Santos TS, Lee JS, Migdal P, Lekshmi IC, Satpati B, Moodera JS. Room-temperature tunnel magnetoresistance and spin-polarized tunneling through an organic semiconductor barrier. PHYSICAL REVIEW LETTERS 2007; 98:016601. [PMID: 17358495 DOI: 10.1103/physrevlett.98.016601] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/04/2006] [Indexed: 05/14/2023]
Abstract
Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.
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Guerrero R, Aliev FG, Tserkovnyak Y, Santos TS, Moodera JS. Shot noise in magnetic tunnel junctions: evidence for sequential tunneling. PHYSICAL REVIEW LETTERS 2006; 97:266602. [PMID: 17280445 DOI: 10.1103/physrevlett.97.266602] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/21/2006] [Indexed: 05/13/2023]
Abstract
We report the experimental observation of sub-Poissonian shot noise in single magnetic tunnel junctions, indicating the importance of tunneling via impurity levels inside the tunnel barrier. For junctions with weak zero-bias anomaly in conductance, the Fano factor (normalized shot noise) depends on the magnetic configuration being enhanced for antiparallel alignment of the ferromagnetic electrodes. We propose a model of sequential tunneling through nonmagnetic and paramagnetic impurity levels inside the tunnel barrier to qualitatively explain the observations.
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